Camptothecins are DNA topoisomerase I inhibitors now being used as anticancer drugs. Topotecan (tpt) and CPT-11 are the first two members in the camptothecin family to gain Food and Drug Administration full approval status (topotecan in 1996 as second-line therapy for advanced epithelial ovarian cancer, topotecan again in 1998 for the treatment of small cell lung cancer, CPT-11 in 1998 as first-line therapy for colon cancer). Several other analogs of the camptothecin family such as GI-147211C, DX8951f, 9-aminocamptothecin (9-AC) and 9-nitrocamptothecin are in various stages of pre-clinical and clinical evaluation. Each of the campothecins in clinical use undergoes relatively rapid hydrolysis in the bloodsteam resulting in a marked loss of anticancer activity. It is the key α-hydroxylactone pharmacophore within clinically relevant camptothecins that hydrolyzes at physiological pH to yield a biologically-inactive and potentially toxic hydroxy carboxylate form. Fassberg, J. and Stella, V. J., “A Kinetic and Mechanistic Study of the Hydrolysis of Camptothecin and Some Analogues”, J. Pharm. Sci. 81: 676–684 (1992); Hertzberg, R. P., Caranfa, M. J., and Hecht, S. M., “On the Mechanism of Topoisomerase I Inhibition by Camptothecin: Evidence for Binding to an Enzyme-DNA Complex”, Biochemistry 28: 4629–4638 (1989); Hsiang, Y-H., and Liu, L. F., “Identification of Mammalian DNA Topoisomerase I as an Intracellular Target of the Anticancer Drug Camptothecin”, Cancer Res. 48: 1722–1726 (1988); and Jaxel, C., Kohn, K. W., Wani, M. C., Wall, M. E., and Pommier, Y., “Structure-Activity Study of Camptothecin Derivatives on Mammalian Topoisomerase I: Evidence for a Specific Receptor Site and a Relation to Antitumor Activity”, Cancer Res. 49: 5077–5082 (1989). References set forth herein, including those set forth above, may facilitate understanding of the present invention. Inclusion of a reference herein is not intended to an does no constitute an admission that the reference is prior art with respect to the present invention.
The structures of camptothecin and some of its important analogs are shown below:

Recent research efforts have shown that agents such as 9-aminocamptothecin and camptothecin (cpt) display very poor stabilities in human blood due to high affinity binding interactions between their carboxylate forms and human serum albumin (HSA). Burke, T. G, Mi, Z., Jiang, Y., and Munshi, C. B. “The Important Role of Albumin in Determining the Relative Human Blood Stabilities of the Camptothecin Anticancer Drugs”, Journal of Pharmaceutical Sciences, 84: 518–519 (1995); Burke, T. G. and Mi, Z. “The Structural Basis of Camptothecin Interactions with Human Serum Albumin: Impact on Drug Stability”, Journal of Medicinal Chemistry, 37: 40–46 (1994); Mi, Z. and Burke, T. G., “Differential interactions of Camptothecin Lactone and Carboxylate Forms with Human Blood Components”, Biochemistry, 33: 10325–10336 (1994); and Mi, Z., Malak, H., and Burke, T. G. “Reduced Albumin Binding Promotes the Stability and Activity of Topotecan in Human Blood”, Biochemistry, 34: 13722–13728 (1995), the disclosures of which are incorporated herein by reference. Frequency-domain lifetime fluorometry experiments revealed that human serum albumin (HSA) preferentially binds camptothecin carboxylate with over a 100-fold higher affinity compared to camptothecin lactone. Mi, Z. and Burke, T. G. “Marked Interspecies Variations Concerning the Interactions of Camptothecin with Serum Albumins: A Frequency-Domain Fluorescence Spectroscopic Study”, Biochemistry, 33: 12540–12545 (1994), the disclosure of which is incorporated herein by reference. This differential binding of carboxylate over lactone results in camptothecin and 9-AC opening more rapidly and completely in the presence of HSA than in the absence of the protein. In human plasma, pH 7.4 and 37° C., camptothecin and 9-AC both open rapidly and essentially completely to almost negligible 0.2% lactone levels at equilibrium. While the presence of HSA promotes lactone ring opening for camptothecin and 9-AC, red blood cells and lipid bilayers in general preferentially bind the electroneutral lactone forms of camptothecins over their respective negatively-charged carboxylate lactone forms. Burke, T. G., Staubus, A. E., Mishra, A. K., and Malak, H., “Liposomal Stabilization of Camptothecin's Lactone Ring”, J. Am. Chem. Soc. 114: 8318–8319 (1992); and Burke, T. G., Mishra, A. K., Wani, M., and Wall, M., “Lipid Bilayer Partitioning and Stability of Camptothecin Drugs”, Biochemistry, 32: 5352–5364 (1993), the disclosures of which are incorporated herein by reference. Drug interactions with erythrocytes thereby promote active lactone levels in blood.